This invention is directed to a heavy medium low specific gravity method for recovering coarse clean coal within a size range of +28 m and fine clean low specific gravity, low sulfur coal and coal-bearing particles within a size range of -28.times.0 m from a raw coal feed having a size range of about 3.81 cm.times.0 and to recover a substantially clean heavy medium which is reusable in the heavy medium low specific gravity system.
It is well-known in the art that coarse clean coal, for example coal having a size of +1.0 mm, can be separated from shale, pyrite, and other impurities in a raw coal feed and can be recovered as a usable product by heavy medium low specific gravity systems using cyclones and screens. When such raw coal feed is cycloned, the constituents in the feed are separated into an overflow fraction containing the relatively light constituents, for example coal and coal-containing particles and an underflow fraction containing the relatively heavier constituents, for example shale and heavy medium. There is little problem in separating the particles in either the overflow and underflow from the heavy medium by screening. However, when the size of the particles becomes smaller, for example less than about 1.0 mm, the separation of the particles becomes progressively more difficult. It is virtually impossible to effect separation of the particles by screening when the particle size is less than about 100 mesh (0.15 mm). Since the requirements for coal quality, i.e., sulfur content is becoming more stringent and the reserves of high grade low sulfur coal are being depleted, there is more emphasis being placed on the development of more efficient heavy medium systems to produce usable coal from all grades and finer sizes of coal.
Several prior art methods to alleviate the above-mentioned problems have been suggested. Indicative of such methods is U.S. Pat. No. 4,140,628 issued Feb. 20, 1979 to David W. Horsfall entitled "Dense Medium Separation". The heavy medium system described by Horsfall is directed to recovering coal which is less than 1000 microns and particularly to coal which is less than 500 microns in size. The patent coal within a size range of 1000 microns is "recalcitrant to separation techniques such as froth-flotation" hence, dense medium separation in a series of cyclones is required. It is obvious that the teaching of Horsfall is directed away from the separation of fine coal and heavy medium by the use of froth-flotation.
U.S. Pat. No. 3,696,923 issued Oct. 10, 1972 to Francis G. Miller entitled "Method for Recovering Fine Coal and Coal-Containing Particles in a Coal Recovery Circuit" teaches that froth-flotation techniques can be used to separate coal from shale in water systems only but not in heavy medium systems.
U.S. Pat. No. 3,794,162 issued Feb. 26, 1974 to Francis G. Miller, et al. and entitled "Heavy Medium Beneficiating Process" teaches the recovery of coarse coal within a size range of 28.times.150 mesh from a raw coal feed in a heavy medium system using magnetite ore as the heavy medium. In the process of the invention, the raw coal to be treated is screened to remove all particles below about 150 mesh prior to combining the raw coal with the heavy medium. Miller, et al. teach that coal particles in the size range of -28.times.0 mesh must be removed from the slurry prior to treatment in a heavy medium system. Hence they do not teach a system which incorporates froth-flotation to recover such fine low sulfur coal, coal-bearing particles and substantially clean heavy medium suitable for reuse in the heavy medium low specific gravity system.
U.S. Pat. No. 2,623,637 issued Dec. 20, 1952 to F. J. Fontein of coal from shale in a raw coal feed in a heavy medium system using cyclones and vibratory screens. The heavy medium is separated from other particles by magnetic separation, hence the heavy medium must be demagnetized prior to reuse in the system. Fontein teaches the use of a series if specially designed cyclones in a plurality of steps to separate the smaller sizes of materials. His teaching is void of the separation of fine coal and coal-bearing particles within a size range of -28.times.0 m and heavy medium in a froth-flotation step wherein the heavy medium is cleaned sufficiently for reuse in the heavy medium low specific gravity systems.
No one nor any combination of the prior art practices shown above teach a heavy medium low specific gravity method in combination with froth-flotation steps wherein coarse coal having a size range of +28 m, fine coal and coal-bearing particles having a size range of -28.times.0 m can be recovered as usable products from a raw coal feed having a particle size range of about 3.81 cm.times.0 and that the heavy medium thus recovered is a substantially clean product suitable for reuse in the heavy medium low specific gravity system. The problems of poor efficiency of coal recovery and the recycling of contaminated heavy medium are thus alleviated.
It is, therefore, the object of this invention to provide a heavy medium low specific gravity method for treating a raw coal feed containing particles within a size range of about 3.81 cm.times.0 wherein coarse coal within the size range of +28 m is recovered as a usable product and the heavy medium is reclaimed as a substantially clean product which can be recycled in the heavy medium low specific gravity system.
It is another object of this invention to provide a heavy medium low specific gravity method for treating a raw coal feed containing particles within a size range of 3.81 cm.times.0 wherein fine gravity low sulfur coal and low sulfur coal-bearing particles within the size range of -28.times.0 m are recovered as usable products and the heavy medium is reclaimed as a substantially relatively clean product in a froth-flotation separation step.
It is also an object of this invention to provide a method for recovering substantially clean heavy medium for reuse in a heavy medium low specific gravity system.